Add a comment for SpyResolver

packages/sol-resolver/src/resolvers/spy_resolver.ts

@@ -4,6 +4,10 @@ import { ContractSource } from '../types';
 
 import { Resolver } from './resolver';
 
+/**
+ * This resolver is a passthrough proxy to any resolver that records all the resolved contracts sources.
+ * You can access them later using the `resolvedContractSources` public field.
+ */
 export class SpyResolver extends Resolver {
     public resolvedContractSources: ContractSource[] = [];
     private readonly _resolver: Resolver;